The present invention relates to a method of on-line monitoring and control of monomer conversion in emulsion polymerization in a reactor, in particular in semicontinuous and continuous emulsion polymerization processes on an industrial scale.
Safety aspects play a prominent role in production processes in chemical industry. Chemical production processes are therefore usually monitored continually in order to avoid possible dangerous situations which could lead to explosions or to release of chemicals.
Many chemical reactions, for example emulsion polymerization, proceed exothermically and are therefore associated with the liberation of heat. If, in such a reaction system, less heat is removed than is generated by reaction of the starting materials the resulting temperature rise in the system can lead to a self-accelerating reaction. This is referred to as a xe2x80x9crunawayxe2x80x9d reaction. In a closed reactor system, a temperature rise is also associated with an increase in the internal pressure in the reactor.
A reactor for exothermic chemical reactions therefore has, in addition to cooling devices for efficient removal of heat, specific safety devices for release of pressure, for example safety valves or special xe2x80x9ccatch tankxe2x80x9d systems which make it possible for the contents of the reactor to be quickly emptied into a safety tank. As a basic safety requirement, the process should always be carried out in such a way that the safety devices are not actuated even under unfavorable conditions, i.e. in the case of a spontaneous, adiabatic runaway reaction of the mixture present in the reactor. To realize this basic principle, reaction monitoring aided by the process control system is usually provided. The essential task of this reaction monitoring is to ensure the safety of the process and to limit the process risk at every point in time during the reaction in the running process.
Up to now, reaction monitoring has usually been based on fixed apparatus-dependent and formulation-independent limit values for feed amounts and/or rates for the starting materials and on monitoring of temperature differences.
These fixed limit values necessitate very large safety margins; optimization of the process conditions in terms of economics is only possible within narrow limits in the case of such concepts.
However, to optimize the space-time yield while maintaining plant safety, it is necessary to replace these rigid limit values by more flexible limit values based on up-to-date measurements while the reaction is running.
In emulsion polymerization, the starting materials (essentially monomers, emulsifiers, water, initiators and stabilizers) are introduced according to a predetermined addition strategy into the reactor where the emulsified monomer droplets are converted into polymer particles with liberation of heat.
Continuous reaction monitoring of emulsion polymerization therefore consists essentially of two elements:
monitoring of a threatened runaway reaction by actuating an alarm if a particular maximum internal temperature in the reactor is exceeded; and
monitoring/actuation of an alarm in the case of monomer accumulation.
An accumulation of monomers in the reactor is, firstly, associated with the risk of the reaction ceasing. However, an accumulation of monomers at the same time also represents an incalculable safety risk should an adiabatic runaway reaction of the reaction mixture occur. Reliable reaction monitoring therefore requires that the reaction enthalpy present in the reactor as a result of accumulative monomers but not yet liberated be known exactly at every point in time.
Various methods of monitoring monomer accumulation are already known.
In the xe2x80x9cde Haasxe2x80x9d reaction monitoring method, the setting of the regulating valves for steam and cooling water supply to the temperature-control bath of the reactor is monitored. This variant has the advantage that it can be implemented relatively simply. It employs instrumentation which is already present for controlling the reaction. However, for this same reason, the method cannot be used as a safety device of requirement class 5 (DIN 19250 or SIL III as per IEC 61508). In addition, certain effects such as reactor fouling or a deterioration in heat removal if the viscosity of the reaction mixture rises cannot be taken into account. The increase in the internal pressure in the reactor which occurs in the case of a runaway reaction is also not taken into account. Furthermore, this method of reaction monitoring reaches its limitations in the case of reactions which are provided with a relatively complex regulation strategy.
A further known method of monitoring monomer accumulation is to monitor the minimum initially charged amount of inerts (for instance deionized water) and the maximum flow for the monomer feed. However, this monitoring method allows only a relatively restricted flexibility with regard to the formulations and the operating procedure for the reactor. In itself it is not sufficient for monitoring the start of the reaction or a cessation of the reaction and must therefore be combined with organizational measures and, if appropriate, the xe2x80x9cde Haasxe2x80x9d reaction monitoring method. This method also does not explicitly take into account pressures which may possibly occur. This method is unfavorable from an economic point of view since, owing to the rigid limit values for amounts, relatively large safety margins have to be allowed.
A further known method is to monitor the temperature difference between the internal reactor temperature and the reactor bath temperature after reaching a xe2x80x9cworst casexe2x80x9d amount. The xe2x80x9cworst casexe2x80x9d amount is the maximum amount of monomers which can be permitted to run into the reactor without occurrence of a polymerization reaction while still leading to conditions within the safety margins in the case of a runaway reaction. The xe2x80x9cworst casexe2x80x9d amount can be determined on the basis of measured flows with the aid of a model. The calculation is then carried out by means of a simplified heat balance which takes into account only the introduced heat flows. However, this method, too, does not explicitly take into account pressures which possibly occur in the case of a runaway reaction. The monitoring of a rigid limit value for the temperature difference between internal reactor temperature and reactor bath temperature does not take into account the influences of reactor fouling and the viscosity. In addition, this method has only restricted usability in the case of reactors having extended cooling opportunities such as external heat exchangers or reflux condensers.
It is an object of the present invention to provide an improved method of on-line monitoring and control of monomer conversion in emulsion polymerization, which method makes possible more economical process conditions combined with unaltered, high plant safety and, in particular, is also usable for reactors having extended cooling opportunities and in processes having complex regulation strategies.
We have found that this object is achieved by the method described in claim 1. The method of the invention comprises
a) selecting an initialization time t0=0 and assigning a particular original heat content Q0 to the reactor for this point in time,
b) as from the initialization time, continuously determining the heat QIN, introduced into the reactor, the reaction enthalpy QRE introduced and the heat QOUT removed from the reactor,
c) calculating the heat which has not been removed QAD according to the following balance
QAD(t)=Q0+QIN(t)+QRE(t)xe2x88x92QOUT(t),
d) calculating the maximum internal temperature TAD which occurs in the case of a spontaneous adiabatic reaction from the heat which has not been removed QAD(t) and the instantaneous internal temperature T(t) of the reactor and,
e) if the calculated maximum internal temperature TAD exceeds the instantaneous internal temperature T(t) of the reactor by a particular margin, implementing measures which prevent a further rise in the heat which is not removed QAD.
The invention thus proposes improving the monitoring of monomer accumulation by introducing an expanded heat balance which takes into account the heat which is removed. The heat which has not been removed QAD, which represents the instantaneous hazard potential, can be determined more accurately and certain parameters relevant to reactor safety, e.g. the maximum adiabatic internal temperature TAD of the reactor, can be calculated more precisely. The safety margins to be maintained before actuation of safety valves or catch tank systems can therefore be better exploited. The continuous determination of the instantaneous conversion and the current hazard potential enables feed rates to be adapted and the space-time yield to be optimized. The heat balance proposed according to the present invention can also be carried out in the case of reactors having external heat exchangers or reflux condensers, since these are technically simple to include in the balance. Moreover, rigid limit values for the maximum feed rate or maximum amounts of starting components do not have to be adhered to. Likewise, rigid limit values for temperature differences between the reactor interior and the bath are no longer necessary. The continuous measurement of the actual amount of heat removed also gives information on reactor fouling or viscosity changes.
A variety of measures are conceivable for preventing a rise in the amount of heat which has not been removed. Preference is given to using one or more of the following measures:
throttling back of the monomer feed,
increasing reactor cooling, for example via the reactor bath or via a reflux condenser,
increasing the initiator addition to achieve better conversion of the accumulated monomers.
Since some of these measures can affect the product quality, particular action strategies influenced by prescribed specifications to be adhered to will be selected on a case-by-case basis.
The heat introduced, the reaction enthalpy and the heat removed from the reactor are advantageously determined by means of temperature and mass flow measurements in the inflow lines and outflow lines of the reactor and in the coolant circuits. Reliable and inexpensive measuring systems are commercially available for temperature and mass flow measurements. If the specific heat capacities of the starting materials are known, the heat flows can be calculated easily. Thus, no relatively large capital costs are associated with the method of the present invention. The method can also be easily implemented in existing plants.
Particular preference is given to additionally calculating the maximum internal pressure pAD prevailing in the reactor at the maximum internal temperature TAD. In this preferred variant of the method of the present invention, the maximum internal pressure in the reactor occurring even in the case of an adiabatic runaway reaction of the mixture in the reactor is employed as an additional safety criterion, so that increased plant safety compared to conventional monitoring methods is obtained. It is therefore advantageous to implement measures which prevent a further rise in the heat which has not been removed QAD if either the calculated maximum internal temperature TAD or the calculated maximum internal pressure pAD exceed the corresponding measured instantaneous values by a particular amount. This variant of the present invention is particularly preferred because pAD is more likely to be exceeded than TAD in the case of a reaction malfunction.
Advantageously, further introduction of monomers into the reactor is completely prevented if the calculated maximum internal temperature TAD and/or the calculated maximum internal pressure pAD exceed prescribed, reactor-specific limit values. As limit values for reaction monitoring, it is possible to employ the design temperature of the reactor or the design pressure of the reactor or of a safety valve which may be present, taking into account the error tolerances of the calculation. With the interruption of the monomer feed, no reaction enthalpy is introduced into the reactor either, so that the conditions prevailing in the reactor are always within the prescribed tolerances. Continued cooling of the reactor then effectively removes heat and, after a certain cooling time, the feed can be reopened if appropriate.
As initialization time for the method of the present invention, a point in time at which the reactor is completely empty is preferably selected and this time is assigned to the original heat content Q0=0.
This initialization can be carried out manually by the operators. However, to avoid human error, the initialization is preferably carried out automatically; for example, after opening the drainage valve for a certain minimum time, it can be assumed that the reactor is completely empty so that automatic initialization can be carried out after this period of time. However, the emptying of the reactor can also be registered via a fill level sensor located in the reactor. The initialization criterion of a completely empty reactor can also be registered by measuring the amount of reaction medium which has run out of the reactor and balancing this with the amount previously fed into the reactor.
In addition, the method of heat balancing according to the present invention also makes it possible to monitor the instantaneous conversion of the reaction mixture and to validate the reactants fed in by calculating the reactor pressure and comparing this with the actual pressure in the reactor. For this reason, preference is given to measuring the actual internal pressure in the reactor p(t) to validate the monitoring method and continuously checking adherence to the relationship
p(t)xe2x89xa6pcalc(t)
where pcalc is the pressure calculated from the instantaneous internal temperature of the reactor.
When the reaction monitoring is installed as a safety device, preference is given to carrying out at least partially redundant temperature, pressure and flow measurements and carrying out continuous validation of the input parameters by comparison of the redundant parameters.
The monitoring method of the present invention can be used particularly advantageously in a semicontinuous emulsion polymerization process. In the case of a continuous polymerization process, the heat removed with the outflowing polymer also has to be included in the heat balance.
For reliable reaction monitoring by means of the heat balance proposed according to the present invention, it is particularly important for the reactor to be very well mixed during the reaction and for the monomers to react uniformly. For this reason, the operation of the reactor stirrer is preferably also monitored continuously. Uniform reaction of the monomers can be ensured in emulsion polymerization by the feed stream procedure.